One reason could be that, not regarding content itself, Tiffany has a near-perfect 'radio' voice: She speaks clearly and smoothly, enunciates well, and, more often than not, has a calming voice. In fact, I wouldn't be surprised if her next career would be in radio itself. Listening to Tiffany is calming and leaves an overall positive effect (however made-up or artificial), even though content is lacking and the positivity oftentimes is a result of her naiveté and lack of education on her specific medical situation.
Nevertheless, critiques on her content that have been written on this forum are mostly valid, given that she has 'cancer' in the titles yet doesn't devote that voice of hers to talk much about it. Basically, I have rationalized the use of 'cancer' in her titles to a situational effect: She had been hospitalized because of surgery required to remove cancerous tumor(s) and cancer-riddled organs, so regardless if she talks about it or not, every clip of her hospital videos is a circumstantial cancer video. For this reason, I give all of her hospital videos a pass on the 'cancer' term in the titles even though, and, especially if she doesn't talk about the cancer: Anyone who clicks on her hospital 'cancer' videos knows that she is in the hospital because of something related to 'cancer' whether or not she talks about it. However, if, for example, she makes videos in the garden center or at home, in a setting that has nothing to do with a direct cancer circumstance (treatment, surgery, doctor's visits, scans), putting 'cancer' in the title is misleading unless she devotes at least half of the video to a discussion of cancer.
Additionally, there is a contradiction of sorts with what she says and how she acts: She is a full-grown adult yet has a child-like nature and mannerisms. Maybe this is part of her real personality or maybe this is accentuated for the videos. People tend to be attracted to rarities, and the Tiffany 'girl-woman' paradox is attention-getting.
Regardless, Tiffany has a way with smoothness and clarity of language for an overall pleasant listening experience, independent of the content itself. Many other you- tubers do not speak as clearly or pleasantly, or have accents which are unpleasant for some people by nature, such as nasal-tone voices and lack of enunciation. Content is much more critical for those types of you- tubers, in my opinion, because their voices alone do not have an attractive factor. Tiffany is the opposite: For me, her voice is currently a positive factor in the views she is getting, though her content is minimal.
Also, a lot of people outside the British empire really do like the British accent: By nature, it sounds refined and elegant, which is attractive in and of itself. (Tiffany definitely has that going for her in the you- tube world!) Perhaps especially for non-Britishers, it is captivating. (Personally, other English accents I find charming are the US Southern accent, and the US Eastern-Boston-type accent--but not at all the US MidWestern 'twangy' nasal-toned voice!) Anyway, voice and how it is projected and sounds can be a factor in a you- tubers popularity.
) channel. 
Maggie's voice and her speech is so much more pleasant to listen to. In addition, she is really smart.
The psychology behind it would be a fascinating and interesting study.
